This invention relates to a spin coating method for forming a thin film on the surface of a semiconductor substrate. It also relates to an apparatus for carrying out this method.
In the manufacture of semiconductor devices, there are many manufacturing steps which require the formation of a thin film on the surface of a semiconductor substrate such as a silicon wafer. For example, in photolithography, which is used to form fine patterns on a silicon wafer, a photosensitive polymeric film called a photoresist is formed on the wafer. In the manufacture of semiconductor imaging devices such as color image sensors, a minute color separation filter called an on-chip filter is formed on a substrate using a certain type of polymer. The most common method for forming thin films of these types is the spin coating method.
In the spin coating method, a substrate which is to be coated is mounted on a chuck which can be rotated by a spindle. The chuck has a support surface which supports the substrate in a horizontal plane. The substrate is generally held rigidly in place on the support surface by suction. A liquid coating material is dripped onto the center of the substrate through a nozzle, and the chuck is then rotated by the spindle at a high speed. Centrifugal force acting on the coating material causes it to spread outwards over the entire surface of the substrate. Excess coating material is flung off the spinning substrate.
The coating material generally contains a solvent. As the spinning continues, the solvent in the coating material vaporizes, and the viscosity of the coating material increases. Gradually, most of the solvent in the coating material vaporizes, the flow of the coating material stops, and a thin film is formed on the substrate.
The manufacture of a semiconductor device requires the formation of a large number of grooves and protrusions in the surface of a substrate. FIG. 25 illustrates a typical silicon wafer 401. A plurality of chips 402 separated by scribe lines 403 are formed on the wafer 401. The scribe lines 403 generally have a depth which is about 1-5 microns less than the thickness of the chips 402, and they have a width on the order of 50-100 microns. When a wafer 401 having surface irregularities such as these scribe lines 403 is subjected to spin coating by conventional methods, the outward flow of the coating liquid is disturbed by the scribe lines 403, and radial irregularities 404 in the film thickness are produced, resulting in a wavy surface. These irregularities 404 develop even when the scribe lines 403 are relatively small, such as on the order of several tenths of a micron up to several microns in size.
When surface irregularities of this type are formed in a photoresist film, they reduce the dimensional accuracy of the pattern which is formed by exposing and developing this film. At times, the irregularities are the cause of breakage of wires in the pattern or of short circuits. Furthermore, when irregularities are formed in an on-chip color filter for a color image sensor, the spectral response characteristics of the filter become nonuniform and vary from filter to filter.
FIGS. 26a through 26c are schematic cross-sectional views of the surfaces of photoresist films formed by spin coating on substrates having three different types of surface irregularities. FIG. 26a shows a pattern of protrusions 502 which is formed on the surface of a substrate 501. The pattern is used for indexing during the exposure stage of photolithography. The surface of a photoresist film 503 which is formed on this pattern contains wave-shaped undulations resulting from the protrusions 502 in the pattern. The peaks 503a and valleys 503b of the surface undulations are not centered over the protrusions 502 and spaces between the protrusions 502. Namely, the center of each peak 503a or valley 503b is slightly deviated from the center of each protrusion 502 or space. This deviation is caused by the flow of the coating material used to form the film 503 during spin coating. In photolithography, the protrusions 502 and spaces in the pattern are detected by a laser beam which is used for indexing an exposure pattern. The asymmetry of the undulations with respect to the protrusions 502 in the pattern has the undesirable effect of displacing the laser beam, and indexing can not be accurately performed. As shown in FIGS. 26b and 26c, even when a pattern consists of only a single protrusion 502 or a single groove 504 in the surface of a substrate 501, the peak 503a or the valley 503b in the surface of the film 503 is still asymmetric and the thickness of the film 503 is different on the left and right sides of the protrusion 502 or groove 504. As a result, dimensional accuracy is decreased. A similar problem occurs when manufacturing on-chip filters for color image sensors by spin coating.
When a film is formed on a substrate by a spin coating method in which the substrate is rotated in a horizontal plane, the only force acting normal to the surface of the substrate is the force of gravity, which is not large enough to remove surface irregularities. Therefore, in order to form a smooth film on a substrate with an irregular surface using a conventional spin coating method, it is necessary either to increase the thickness of the film or to form a plurality of films, as in the multi-layer resist method. However, both of these methods are inefficient ways to form a smooth film.